The function of the butterfly needle is draw blood samples from wrist, hand and foot , its benefits really appear when the veins are too deep or when uncooperative patients keep moving or is anxious during blood drawn

Guidelines For Butterfly Needle Gauge Selection

When we want to draw blood and the regular straight needle is not ideal we use the butterfly needle for this job , the butterfly needle is indicated in some situations as when the patient is anxious , can't stay still , to allow the ability of movement during the drawing process and it is specially used for patients with deep or thin veins.

The butterfly needle differs from the straight ordinary needle in some points , as butterfly needle is shorter having rubber wings on both sides to provide a good grip during handling and at the end it is connected to the collecting tubes.

Butterfly needles comes in different gauges 21 , 23 , 25 where here by increasing gauge number the diameter decreases , but choosing the right gauge for a specific patient follows some guidelines

Size of patients veins: higher gauge needles work
better for pediatric or geriatric patients in many cases. but, care
should be taken to avoid too much vacuum when drawing or pressure
when infusing as it can have harmful side effects and even render a sample
useless.

Speed of infusion or draw: thinner needles move less
fluid that thicker needles. Depending on time requirements or the
equipment and tubing used, particular butterfly needle sizes might be
required.

Specific medication requirements: some medications
are thicker and more viscous. This requires a larger diameter needle to
properly infuse the medication without causing complications with
clotting or pressure in the loop.